1. Field of the Invention
The present invention relates to a dosing device and in particular to a dosing device suitable for use in an anaesthesia machine or ventilator to regulate the concentration of a liquid vapor, such as an anaesthetic vapor, in a carrier gas component of a breathing gas.
2. Description of the Prior Art
It is common to administer liquid anaesthetic agents to a patient as a gas. This is done by vaporizing the liquid and then conducting the vaporized anaesthetic to the patient in a carrier gas. It is important that the amount of anaesthetic agent is accurately controlled and this is generally achieved in one of two ways; either by controlling the flow rate of the carrier gas through a reservoir of liquid anaesthetic or by controlling the dose of liquid anaesthetic to be supplied for vaporization. If a fault occurs in a device operating in the first way then relatively high doses can be taken from the reservoir. Even if other security measures for the patient are provided and are triggered, the patient may still be exposed to a quantity that could cause discomfort, and in the worst case even a harmful or lethal quantity.
A known dosing device for controlling the dose of liquid anaesthetic to be supplied for vaporization is described in WO 92/19303 (BOC Group PLC) and has a variable volume pump which can be operated to define the dose to be supplied by varying the volume in a controlled manner using a stepper motor. This device, however, requires that the pump and stepper motor are manufactured to a high mechanical tolerance, which adds to the cost of such a device as well as to the manufacturing difficulties. Additionally the generation of an intermittent flow, particularly of low flow rate, may be problematic since this requires accurate control of the pump volume over the majority of its stroke length.